1. Field of the Invention
The present invention relates to a steering system, in which an operating element that is to be operated by a driver, is not mechanically connected to steered road-wheels to be steered or rotated to steer a vehicle.
2. Description of the Background Art
In a so-called SBW (steer-by-wire) type steering system in which a steering wheel (an operating element) that is to be operated by the driver, and steered road-wheels are not mechanically connected to each other, the steered road-wheels are turned by a steering motor and a reaction force is imparted to the operating element by a reaction motor. These motors are controlled independently, whereby a ratio of a rotating angle of the steered road-wheels to a steering input to the operating element is varied depending on vehicle speeds to thereby largely contribute to the stabilization of the behaviors of a vehicle.
In this SBW type steering system, since it is only the drive force of the steering motor that turns the steered road-wheels with no steering effort that is inputted to the operating element by the driver imparted to the steered road-wheels, the steering motor of the SBW type steering system needs a larger output than that of a steering motor for a steering system in which the operating element and the steered road-wheels are mechanically connected (hereinafter, referred to as an electric power steering).
Consequently, even compared in the same driving conditions, more heat is generated in the steering motor and a power drive unit which controls the drive of the steering motor in the SBW type steering system than in the electric power steering. Due to this, in such a situation that a large load is applied to a rotating system as when the steered wheels are rotated about their swivel pins at extremely low speeds or during stationary steering, the heat generation in a steering system needs to be suppressed.
Japanese Patent Unexamined Publication No. JP-A-2004-194385 discloses a technique to reduce power supplied to the reaction motor when the temperature of the reaction motor reaches or exceeds a predetermined temperature.
However, when the supply of power to the reaction motor is reduced when the temperature of the reaction motor reaches or exceeds the predetermined temperature as described the JP-A-2004-194385, while the increase in the temperature of the reaction motor can be suppressed, since the reaction force is reduced, the driver is then allowed to increase the steering input to the operating element with ease, and as a result, the load to the steering motor is increased, causing a problem that the generation of heat in the steering motor is promoted. Note that in the SBW type steering system, the steering motor is generally required to have a larger output than the reaction motor.